Study On Solid State Reaction Synthesis By Two-step Calcination And Carbon-coated Modification Of Li₄Ti₅O₁₂ Electrode Material

Lithium ion battery may be one of the most promising power source of electric vehicle as its high energy density, high potential and nonmemeory effect as well. Li-ion intercalated compound Li₄Ti₅O₁₂ has been demonstrated to be one of the most promising electrode materials for such applications, because it has a flat voltage range, high reversible capacity(175mAh·g^-1), and especially the long cycling performance due to no structural change zero-strain insertion material during charge-discharge cycling, besides of these, titanium is inexpensive and abundantly existing in nature.However, the drawback of its poor electrical conductivity(10^-13 S·cm^-1) and low Li-ion diffuse coefficient (2×10^-13cm²Â·s^-1) caused its poor rate capability. Considering of promoting the industry producing of Li₄Ti₅O₁₂, it's synthesized by solid reaction, and the usual saccharide was used to improve its rate capability. The main research results and conclusion are as follows:(1) The calcined system of synthesizing Li₄Ti₅O₁₂ was investigated through solid reation at high temperature. The result showed that the pure phase spinel Li₄Ti₅O₁₂ can be obtained when two-step calcination system at low temperature was used. The detailed process was: The raw materials of Li₂CO₃ and TiO₂ with a ratio of Li/Ti was excessive by 8% were pre-calcined at 650℃for 8h then continued were treated at 800℃for 6h. The product has a first discharge specific capacity at 168.7mAh·g^-1, and a discharge-capacity of 120.6mAh·g^-1 at 0.5C charge-discharge rate while the first discharge-capacity was 80.6mAh·g^-1 at 1C charge-discharge cycle which has an improved rate capability compared with impurity containing product.(2) The carbon modification using glucose as carbon source was researched. The effect of carbon content and carbonized time on the Li₄Ti₅O₁₂/C composite material structure and electrochemical properties was investigated. The sample with 2% carbon(mass, aim content in Li₄Ti₅O₁₂/C) which was kept at 800℃for 4 hours in N₂ gas displayed the best electrical performance, a 154.3 mAh·g^-1 of initial specific capacity was obtained at 0.1C, while 148. 8 mAh·g^-1 at 0.2C and the specific capacity keep rate improved by 40% compared with the raw Li₄Ti₅O₁₂. Based on the result, the effect of the different carbon sources such as glucose, saccharose and starch on the carbon modification was studied. The result indicated that starch has the best effect to improve Li₄Ti₅O₁₂ eletrochemical propeties at high current followed by saccharose. When starch was used as carbon source, the composite material of Li₄Ti₅O₁₂/C has an initial discharge rate capacity of 159mAh·g^-1 and even a specific capacity of 125mAh·g^-1 at 2C.(3) The Li₄Ti₅O₁₂/C composite materials was synthesized by intermediate ball-milled method whose carbon sources were glucose, saccharose and starch. XRD results showed that these Li₄Ti₅O₁₂/C composite materials had a high purity of Li₄Ti₅O₁₂, which meant that the problem that the products contained many impurities gotten through primary pre-calcined system was been resolved. The results showed that the Li₄Ti₅O₁₂/C composite materials using starch as carbon source has an excellent eletrochemical propeties. At 0.1C,1C and 2C charge-discharge rate, the composite material has initial discharge specific capacity of 162, 146.5 and 136mAh·g^-1, respectively. The capacity keep rate at 1C was two times with the raw materials and 85% at 2C.